1. Field of the Invention
The present invention relates to a raw material feeding apparatus, such as an injection molding machine or an extruding molding machine, and more particularly to such type of raw material feeding apparatus which is excellent for use in a color-change or other resin-change (hereinafter called simply "resin-change") process.
2. Description of the Prior Art:
One example of an improved prior art raw material feeding apparatus for a resin-change process in an injection molding machine or an extrusion molding machine is disclosed in Japanese Patent Publication No. 62-41450 (1987). In this system, as shown in FIG. 8, a shutter 8 is provided midway along a raw material passageway 9 in the raw material feeding system, and raw material resin is intermittently fed by controllably opening and closing this shutter.
In this figure, reference numeral 1 designates a screw, numeral 2 designates a cylinder, numeral 3 designates an end cap, numeral 4 designates an injection nozzle, numeral 5 designates a hydraulic motor for rotating the screw 1, numeral 6 designates a hopper for feeding raw material resin, and numeral 7 designates an injection cylinder. The shutter 8 is provided for opening and closing the raw material passageway 9 at predetermined times, and is controlled by a controller (not shown). It is to be noted that in this figure, reference numeral 10 designates a torpedo-shaped tip member.
A feed section a, a compression section b, and a metering section c are sequentially formed along the screw 1 from its base end portion. The locations where old resin is most liable to collect are an outer circumferential surface d of the tip member 10, an inner wall surface e of the end cap 3, and an inner wall surface f of the nozzle 4. Here, the raw material feeding system includes the hopper 6, the shutter 8, the raw material passageway 9 and a shutter opening/closing mechanism (not shown). It is to be noted that the raw material passageway 9 includes not only a portion of the cylinder 2, but also a cylindrical outlet portion of the hopper 6.
The method of resin-change for the raw material being fed in the above-described raw material feeding system as disclosed in the above-noted Japanese patent publication consists of the following steps:
(1) At first, the raw material passageway 9 in the raw material feeding apparatus is blocked by the shutter 8 to interrupt feeding of old raw material. Under this condition, rotation of the screw 1 and injection are repeated to bring at least the feed section a and the compression section b of the screw 1 into a hollow or emptied condition (this is also called "hunger condition"), and also the raw material within the hopper 6 is replaced by new material.
(2) Next, the shutter 8 is opened and the screw 1 is rotated. At this time, the above-mentioned hollow section is filled with new non-molten resin.
(3) Under this condition, if rotation of the screw 1 is continued, molten resin having a higher viscosity than at the time of steady operation is fed to the resin collecting sections d, e and f in the tip end portion of the screw 1. Therefore, the resin pressure increases and the screw 1 is caused to retreat.
(4) When the screw 1 has retreated to the extreme rearward end of a metering stroke, rotation of the screw 1 is stopped, a hydraulic pressure is applied to the back pressure side of the cylinder 7, and the screw 1 advances to cause injection. At this time, molten resin having a high viscosity flows through the residual resin collecting sections d, e and f, such that a large shearing force acts upon the wall surface of the resin passageway and scrapes the old residual resin adhered to the wall surface. By repeating these screw rotation and injection steps, removal of old residual resin adhered to various surfaces proceeds to thereby result in change of the resin such that the removal soon reaches a steady state. This is caused, in the upstream portion of the resin flow, mainly by a scraping action caused by shearing forces exerted by a solid bed of non-molten resin, and in the downstream portion of the same, by a scraping action caused by large shearing forces due to flow of high viscosity molten resin.
(5) However, with this newly established steady state, temperatures at the respective portions will increase and viscosity of the resin will decrease, such that the shearing forces acting upon the wall surfaces are reduced and the above-described effect decreases. Therefore, the shutter 8 is closed, and the above-described respective operations (1), (2) and (3) can again be carried out. These operations can be carried out repeatedly.
In the above-described prior art method of resin-change, feeding of raw material resin within a hopper is stopped and new non-molten resin is fed to a hollow (or empty) space within a cylinder where old molten resin has been extracted. This provides special advantages in terms of reduction of the amount of consumed resin by 60% as compared to the conventional method of the prior art, in which resin change is effected simply by repeating rotation of a screw and injection after replacement of resin.
However, a shortcoming remains in the above-described method of resin-change, in that the time required for resin-change is not improved significantly relative to the conventional method in the prior art. The principal cause of this resides in the fact that the amount of resin filled per cycle in the above-described resin-change operation steps (1)-(5) is too large, such that after it has reached the steady state, the time necessary for producing a hollow or empty space to fill with non-molten resin, that is, the time for discharging the resin, cannot be shortened.